(a) Field of the Invention
This invention relates to an apparatus for measuring the blood pressure automatically, continuously and indirectly.
(b) Description of the Prior Art
It is known in the art that the blood pressure can be measured by applying the external pressure to an artery to be measured by an occluding cuff. This method is referred to as an auscultatory method. In this case, if the external pressure is higher than the systolic blood pressure, the bloodflow in the distal part of the cuff does not exist, and if the external pressure is kept between the systolic and diastolic blood pressure, the detection of Korotkoff sounds can be obtained by a proper transducer which is placed in the distal end of the cuff. Futhermore, if the external pressure is lower than the diastolic blood pressure, even though the bloodflow exists, its sound is not heard or is very weak. Therefore, the generally used sphygmomanometer can continuously measure only one of the systolic or diastolic blood pressures.
However, the need to measure both the systolic and diastolic blood pressures for every heart beat could not be fully satisfied, if an instrument which obtains only either one of the systolic and diastolic blood pressures, is used.
In view of the above problem, the applicant has proposed an indirectly and instantaneously measurable sphygmomanometer and filed on Mar. 23, 1979 at the U.S. Patent Office under the Ser. No. 023,267, now abandoned. This apparatus uses a volume compensation method and enables to measure continuously both the systolic and diastolic blood pressures as well as the instant blood pressure waveform. The method is based on the indirect unloading of the vascular wall, i.e., the vascular volume per unit length of blood vessel is maintained constant by applying the external pressure thereto. The applied external cuff pressure is controlled to balance, with the intravascular pressure or arterial blood pressure, so that the cuff pressure indirectly shows the arterial pressure.
The above sphygmomanometer has been found, however, not satisfactorily in that the operation and adjustment for obtaining the continuous blood pressure is done manually and it is very complicate to operate, and moreover it requires fine adjustment for each different subject or person.